Take a round toothpick and put a drop of water on your coin at 6 OC. Touch the drop of water down to the spot and see if it dissolves.
Not finning - I can't tell if it's just extra schmootzem - hard to tell from the photo because that 'stuff' looks much lighter than the coins surfaces or rims - I think 'something' was on the coin, and I'd guess it was after the coin was struck. It doesn't look like it was there when the coin was struck, based on what I can see in the photo A few (few) more photos, closer up, maybe a slightly different angle, might be helpful.
Thanks! water didn’t dissolve it. But probably you guys are right and it’s some kind of dirt or pmd. The coin is AU through.
Check out the inside rim at 1 o'clock. Sometimes, when a coin is struck, a thin layer of metal is smashed down next to the rim. It is the same color as the coin. Now look at the other parts of the rim where what looks like a "rounded residue" of a different color is present. I believe something will dissolve it away. If it is a deposit of some kind, I could remove it very easily but I need to be careful giving advice how to do it so the coin is not damaged. Gently push down on the off color area with your fingernail.